Network Working Group F. Baker, Editor
Request for Comments: 1220 ACC
April 1991
Point-to-Point Protocol Extensions for Bridging1. Status of this Memo
This document defines an extension of the Internet Point-to-Point
Protocol (PPP) described in RFC 1171, targeting the use of Point-to-
Point lines for Remote Bridging. It is a product of the Point-to-
Point Protocol Extensions Working Group of the Internet Engineering
Task Force (IETF).
This RFC specifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
2. Historical Perspective
Two basic algorithms are ambient in the industry for Bridging of
Local Area Networks. The more common algorithm is called
"Transparent Bridging" and has been standardized for Extended LAN
configurations by IEEE 802.1. IEEE 802.5 has proposed an alternative
approach, called "Source Routing", and is in the process of
standardizing that approach for IEEE 802.5 extended networks.
Although there is a subcommittee of IEEE 802.1 addressing remote
bridging, neither standard directly defines Remote Bridging per se,
as that would technically be beyond the IEEE 802 committee's charter.
Both allow for it, however, modeling the line as an unspecified
interface between half-bridges.
This document assumes that the devices at either end of a serial link
- have agreed to utilize the RFC 1171 line discipline in some form.
- may have agreed, by some other means, to exchange other
protocols on the line interspersed with each other and with any
bridged PDUs.
- may be willing to use the link as a vehicle for Remote Bridging.
- may have multiple point-to-point links that are configured in
parallel to simulate a single line of higher speed or
Point-to-Point Protocol Extensions Working Group [Page 1]

RFC 1220 Bridging Point-to-Point Protocol April 1991
reliability, but message sequence issues are solved by the
transmitting end.
3. General Considerations3.1. Link Quality Monitoring
It is strongly recommended that Point-to-Point Bridge Protocol
implementations utilize Magic Number Loopback Detection and Link-
Quality-Monitoring. This is because the 802.1 Spanning Tree
protocol, which is integral to both Transparent Bridging and Source
Routing (as standardized), is unidirectional during normal operation,
with HELLO PDUs emanating from the Root System in the general
direction of the leaves, without any reverse traffic except in
response to network events.
3.2. Message Sequence
The multiple link case requires consideration of message
sequentiality. The transmitting station must determine either that
the protocol being bridged requires transmissions to arrive in the
order of their original transmission, and enqueue all transmissions
on a given conversation onto the same link to force order
preservation, or that the protocol does NOT require transmissions to
arrive in the order of their original transmission, and use that
knowledge to optimize the utilization of the several links, enqueuing
traffic to links to minimize delay.
In the absence of such a determination, the transmitting station must
act as though all protocols require order preservation; many
protocols designed primarily for use on a single LAN in fact do. A
protocol could be described to maintain message sequentiality across
multiple links, either by sequence numbering or by fragmentation and
re-assembly, but this is neither elegant nor absolutely necessary.
3.3. Maximum Receive Unit Considerations
Please note that the negotiated MRU must be large enough to support
the MAC Types that are negotiated for support, there being no
fragmentation and re-assembly. Even Ethernet frames are larger than
the default MRU of 1500 octets.
3.4. Separation of Spanning Tree Domains
It is conceivable that a network manager might wish to inhibit the
exchange of BPDUs on a link in order to logically divide two regions
into separate Spanning Trees with different Roots (and potentially
different Spanning Tree implementations or algorithms). In order to
Point-to-Point Protocol Extensions Working Group [Page 2]

RFC 1220 Bridging Point-to-Point Protocol April 1991
do that, he must configure both ends to not exchange BPDUs on a link.
For the sake of robustness, a bridge which is so configured must
silently discard the BPDU of its neighbor, should it receive one.
4. IEEE 802.1 Transparent Bridging4.1. Overview of IEEE 802.1 Transparent Bridging
As a favor to the uninitiated, let us first describe Transparent
Bridging. Essentially, the bridges in a network operate as isolated
entities, largely unaware of each others' presence. A Transparent
Bridge maintains a Forwarding Database consisting of
{address, interface}
records by saving the Source Address of each LAN transmission that it
receives along with the interface identifier for the interface it was
received on. It goes on to check whether the Destination Address is
in the database, and if so, either discards the message (if the
destination and source are located at the same interface) or forwards
the message to the indicated interface. A message whose Destination
Address is not found in the table is forwarded to all interfaces
except the one it was received on; this describes Broadcast/Multicast
behavior as well.
The obvious fly in the ointment is that redundant paths in the
network cause indeterminate (nay, all too determinate) forwarding
behavior to occur. To prevent this, a protocol called the IEEE
802.1(d) Spanning Tree Protocol is executed between the bridges to
detect and logically remove redundant paths from the network.
One system is elected as the "Root", which periodically emits a
message called a Bridge Hello Protocol Data Unit, or BPDU, heard by
all of its neighboring bridges. Each of these modifies and passes
the BPDU on to its neighbors, and they to theirs, until it arrives at
the leaf LAN segments in the network (where it dies, having no
further neighbors to pass it along) or until the message is stopped
by a bridge which has a superior path to the "Root". In this latter
case, the interface the BPDU was received on is ignored (i.e., it is
placed in a Hot Standby status, no traffic is emitted onto it except
the BPDU, and all traffic received from it is discarded) until a
topology change forces a recalculation of the network.
4.2. IEEE 802.1 Remote Bridging Activity
There exist two basic sorts of bridges - ones that interconnect LANs
directly, called Local Bridges, and ones that interconnect LANs via
an intermediate medium such as a leased line, called Remote Bridges.
Point-to-Point Protocol Extensions Working Group [Page 3]

RFC 1220 Bridging Point-to-Point Protocol April 1991
The Point-to-Point Protocol might be used by a Remote Bridge.
There is more than one proposal within the IEEE 802.1 Interworking
Committee for modeling the Remote Bridge. In one model, the
interconnecting serial link(s) are treated in the same way that a LAN
is, having a standard IEEE 802.1 Link State; in another, the serial
links operate in a mode quite different from the LANs that they
interconnect. For the sake of simplicity of specification, the first
model is adopted, although some of the good ideas from proponents of
the second model are included or allowed for.
Therefore, given that transparent bridging is configured on a line or
set of lines, the specifics of the link state with respect to the
bridge is defined by IEEE 802.1(d). The Bridge Protocol Data Unit,
or BPDU, is defined there, as well as the algorithms for its use.
It is assumed that, if a Point-to-Point Link neighbor receives IEEE
802.1 BPDUs without rejecting them with the RFC 1171 Protocol-Reject
LCP PDU, Transparent Bridging is permitted on the link.
4.3. IEEE 802.5 Source Routing
The IEEE 802.5 Committee has defined a different approach to bridging
for use on the Token Ring, called Source Routing. In this approach,
the originating system has the responsibility of indicating what path
that the message should follow. It does this, if the message is
directed off the local ring, by including a variable length MAC
header extension called the Routing Information Field, or RIF. The
RIF consists of one 16 bit word of flags and parameters followed by
zero or more ring-and-bridge identifiers. Each bridge en route
determines from this "source route list" whether it should receive
the message and how to forward it.
The algorithm for Source Routing requires the bridge to be able to
identify any interface by its ring-and-bridge identifier, and to be
able to identify any of its OTHER interfaces likewise. When a packet
is received which has the Routing Information Field (RIF) present, a
boolean in the RIF is inspected to determine whether the ring-and-
bridge identifiers are to be inspected in "forward" or "reverse"
sense. In a "forward" search, the bridge looks for the ring-and-
bridge identifier of the interface the packet was received on, and
forwards the packet toward the ring identified in the ring-and-bridge
identifier that follows it. In a "reverse" search, the bridge looks
for the ring-and-bridge identifier of the OTHER INTERFACE, and
delivers the packet to the indicated interface if such is found.
The algorithms for handling multicasts ("Functional Addresses" and
"Group Addresses") have been the subject of much discussion in 802.5,
Point-to-Point Protocol Extensions Working Group [Page 4]

RFC 1220 Bridging Point-to-Point Protocol April 1991
and are likely to be the most troublesome for bridge implementations.
Fortunately, they are beyond the scope of this document.
4.4. IEEE 802.5 Remote Bridging Activity
There is no Remote Bridge proposal in IEEE 802.5 at this time,
although IBM ships a remote Source Routing Bridge. Simplicity would
dictate that we choose the same model for IEEE 802.5 Source Routing
that was selected for IEEE 802.1, but necessity requires a ring
number for the line in some cases. We allow for both models.
Given that source routing is configured on a line or set of lines,
the specifics of the link state with respect to the bridge is defined
by the IEEE 802.5 Addendum on Source Routing. The requisite PDUs for
calculating the spanning tree (used for assuring that each ring will
receive at most one copy of a multicast) are defined there, as well
as the algorithms for their use. MAC PDUs (Beacon, Ring Management,
etc) are specific to the MAU technology and are not exchanged on the
line.
4.5. Source Routing to Transparent Bridge Translation
IEEE 802 also has a subcommittee looking at the interoperation of
Transparent Bridging and Source Routing. For the purposes of this
standard, such a device is both a transparent and a source routing
bridge, and will act on the line in both ways, just as it does on the
LAN.
5. Traffic Services
Several services are provided for the benefit of different system
types and user configurations. These include LAN Frame Checksum
Preservation, LAN Frame Checksum Generation, Tinygram Compression,
and the identification of closed sets of LANs.
5.1. LAN Frame Checksum Preservation
IEEE 802.1 stipulates that the Extended LAN must enjoy the same
probability of undetected error that an individual LAN enjoys.
Although there has been considerable debate concerning the algorithm,
no other algorithm has been proposed than having the LAN Frame
Checksum received by the ultimate receiver be the same value
calculated by the original transmitter. Achieving this requires, of
course, that the line protocols preserve the LAN Frame Checksum from
end to end. The protocol is optimized towards this approach.
Point-to-Point Protocol Extensions Working Group [Page 5]

RFC 1220 Bridging Point-to-Point Protocol April 19915.2. Traffic having no LAN Frame Checksum
The fact that the protocol is optimized towards LAN Frame Checksum
preservation raises twin questions: "What is the approach to be used
by systems which, for whatever reason, cannot easily support Frame
Checksum preservation?" and "What is the approach to be used when the
system originates a message, which therefore has no Frame Checksum
precalculated?".
Surely, one approach would be to require stations to calculate the
Frame Checksum in software if hardware support were unavailable; this
would meet with profound dismay, and would raise serious questions of
interpretation in a Bridge/Router.
However, stations which implement LAN Frame Checksum preservation
must already solve this problem, as they do originate traffic.
Therefore, the solution adopted is that messages which have no Frame
Checksum are tagged and carried across the line.
When a system which does not implement LAN Frame Checksum
preservation receives a frame having an embedded FCS, it converts it
for its own use by removing the trailing four octets. When any
system forwards a frame which contains no embedded FCS to a LAN, it
forwards it in a way which causes the FCS to be calculated.
5.3. Tinygram Compression
An issue in remote Ethernet bridging is that the protocols that are
most attractive to bridge are prone to problems on low speed (64 KBPS
and below) lines. This can be partially alleviated by observing that
the vendors defining these protocols often fill the PDU with octets
of ZERO. Thus, an Ethernet or IEEE 802.3 PDU received from a line
that is (1) smaller than the minimum PDU size, and (2) has a LAN
Frame Checksum present, must be padded by inserting zeroes between
the last four octets and the rest of the PDU before transmitting it
on a LAN. These protocols are frequently used for interactive
sessions, and therefore are frequently this small.
To prevent ambiguity, PDUs requiring padding are explicitly tagged.
Compression is at the option of the transmitting station, and is
probably performed only on low speed lines, perhaps under
configuration control.
The pseudo-code in Figure 1 describes the algorithms.
Point-to-Point Protocol Extensions Working Group [Page 6]

RFC 1220 Bridging Point-to-Point Protocol April 19915.4. LAN Identification
In some applications, it is useful to tag traffic by the user
community it is a part of, and guarantee that it will be only emitted
onto a LAN which is of the same community. The user community is
defined by a LAN ID. Systems which choose to not implement this
feature must assume that any frame received having a LAN ID is from a
different community than theirs, and discard it.
Point-to-Point Protocol Extensions Working Group [Page 7]

RFC 1220 Bridging Point-to-Point Protocol April 1991
number tells the receiving system how many octets to strip off the
end.
MAC Type:
0: Reserved
1: IEEE 802.3/Ethernet
2: IEEE 802.4
3: IEEE 802.5
4: FDDI
other: Assigned by the Internet Assigned Numbers Authority
LAN ID:
This optional 32 bit field identifies the Community of LANs which
may be interested to receive this frame, as described in section5.4. If the LAN ID flag is not set, then this field is not
present, and the PDU is four octets shorter.
Frame Control:
On 802.4, 802.5, and FDDI LANs, there are a few octets preceding
the Destination MAC Address, one of which is protected by the FCS.
Since the MAC Type field defines the bit ordering, these are sent
in MAC order. A pad octet is present to avoid odd machine address
boundary problems.
Destination MAC Address:
As defined by the IEEE. Since the MAC Type field defines the bit
ordering, this is sent in MAC order.
Source MAC Address:
As defined by the IEEE. Since the MAC Type field defines the bit
ordering, this is sent in MAC order.
LLC data:
This is the remainder of the MAC frame. This is that portion of
the frame which is (or would be were it present) protected by the
LAN FCS; for example, the 802.5 Access Control field, and Status
Trailer are not meaningful to transmit to another ring, and are
omitted.
LAN Frame Checksum:
If present, this is the LAN FCS which was calculated by (or which
appears to have been calculated by) the originating station. If
the FCS Present flag is not set, then this field is not present,
and the PDU is four octets shorter.
Optional Data Link Layer Padding
RFC 1171 specifies that an arbitrary pad can be added after the
data intended for transmission. The "Count" portion of the flag
Point-to-Point Protocol Extensions Working Group [Page 11]

RFC 1220 Bridging Point-to-Point Protocol April 19916.3. IEEE 802 Network Control Protocol
The Bridge Network Control Protocol is responsible for configuring,
enabling, and disabling the bridges on both ends of the point-to-
point link. As with the Link Control Protocol, this is accomplished
through an exchange of packets. BNCP packets may not be exchanged
until LCP has reached the network-layer Protocol Configuration
Negotiation phase. Likewise, LAN traffic may not be exchanged until
BNCP has first opened the connection.
The Bridge Network Control Protocol is exactly the same as the Point-
to-Point Link Control Protocol with the following exceptions:
Data Link Layer Protocol Field
Exactly one Bridge Network Control Protocol packet is encapsulated
in the Information field of PPP Data Link Layer frames where the
Protocol field indicates type hex 8031 (BNCP).
Code field
Only Codes 1 through 7 (Configure-Request, Configure-Ack,
Configure-Nak, Configure-Reject, Terminate-Request,
Terminate-Ack and Code-Reject) are used. Other Codes should
be treated as unrecognized and should result in Code-Rejects.
Timeouts
BNCP packets may not be exchanged until the Link Control
Protocol has reached the network-layer Protocol Configuration
Negotiation phase. An implementation should be prepared to wait
for Link Quality testing to finish before timing out waiting
for Configure-Ack or other response.
Configuration Option Types
The Bridge Network Control Protocol has a separate set of
Configuration Options. These permit the negotiation of the
following items:
- MAC Types supported
- Tinygram Compression support
- LAN Identification support
- Ring and Bridge Identification
6.4. IEEE 802.5 Remote Ring Identification Option
Since the Remote Bridges are modeled as normal Bridges with a strange
internal interface, each bridge needs to know the ring/bridge numbers
of the bridges it is adjacent to. This is the subject of a Link
Negotiation. The exchange of ring-and-bridge identifiers is done
using this option on the Network Control Protocol.
Point-to-Point Protocol Extensions Working Group [Page 13]

RFC 1220 Bridging Point-to-Point Protocol April 1991
Type 2 = IEEE 802.5 Line "Ring/Bridge" Identifier
Length
4 Octets
Ring Number
A 12 bit number identifying the line, as defined in the
IEEE 802.5 Source Routing Specification.
Bridge Number
A 4 bit number identifying the bridge on the token ring, as
defined in the IEEE 802.5 Source Routing Specification.
6.6. MAC Type Support Selection
The MAC Type Selection Option is provided to permit nodes to
advertise what sort of traffic they are prepared to convey. A device
negotiating a 1600 octet MRU, for example, may not be willing to
support 802.5 (although it might, with certain changes necessary in
the RIFs it passes, and given that the hosts it supports implement
the 802.5 Maximum Frame Size correctly), and is definitely not
prepared to support 802.4 or FDDI.
A system which does not announce the MAC Types that it supports may
be assumed to support all MAC Types; it will discard those that it
does not understand. A system which chooses to announce MAC Types is
advising its neighbor that all unspecified MAC Types will be
discarded. Announcement of multiple MAC Types is accomplished by
placing multiple options in the Configure Request.
The Rejection of a MAC Type Announcement (in a Configure-Reject) is
essentially a statement that traffic appropriate to the MAC Type, if
encountered, will be forwarded on the link even though the receiving
system has indicated it will discard it.
Figure 7: MAC Type Selection Option
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| type=3 |length = 3 | MAC Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 3 = MAC Type Selector
Length
3 Octets
Point-to-Point Protocol Extensions Working Group [Page 15]

RFC 1220 Bridging Point-to-Point Protocol April 1991
MAC Type Selector
One of the values of the PDU's MAC Type Field that this system is
prepared to receive and service.
6.7. Tinygram Compression
Not all systems are prepared to make modifications to messages in
transit; on high speed lines, it is probably not worth the effort.
This option permits the system to negotiate compression.
Consistent with the behavior of other compression options in the
Internet Point-to-Point set of protocols, no negotiation implies no
compression. The systems need not agree on the setting of this
parameter; one may be willing to decompress and the other not. A
system which does not negotiate, or negotiates this option to be
disabled, should never receive a compressed packet, however.
Figure 8: Tinygram Compression Option
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| type=4 |length = 3 | Compression |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 4 = Tinygram Compression Support Option
Length
3 Octets
Compression Enable/Disable
If the value is 1, Tinygram Compression is enabled. If the
value is 2, Tinygram Compression is disabled, and no
decompression will occur.
6.8. LAN Identification Support
Not all systems are prepared to make use of the LAN Identification
field. This option enables the systems to negotiate its use.
The parameter is advisory; if the value is "enabled", then there may
exist labeled LANs beyond the system, and the system is prepared to
service traffic to it. if the value is "disabled", then there are no
labeled LANs beyond the system, and all such traffic will by
definition be dropped. Therefore, a system which is advised that his
peer does not service LAN Identifications need not forward such
traffic on the link.
Point-to-Point Protocol Extensions Working Group [Page 16]